Aaron Kromer's Blog

Recently I was looking for a way to do a comparison on a String with either another String or a Regexp. Most of the discussions on equality focused on ==, eql?, equal?. None of which would satisfy the requirement. So I was left with this code:

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def matches(compare_with)
  if compare_with.is_a?(Regexp)
    @data_string =~ compare_with
  else
    @data_string == compare_with
  end
end

I was less than thrilled. So I did what everyone does, I asked the internet. Thanks to Twitter, specifically James Edward Gray II @JEG2 who btw completely rocks, I was pointed at ===. Though the documentation on === leaves something to be desired:

Used to compare each of the items with the target in the when clause of
a case statement.

• The String API sneakily directs you to == but doesn’t outright state they are the same
• The Regexp API states it as a synonym for Regexp#=~

The thing to remember is with case when you have the following:

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case thing
when other_thing
  # stuff
end

You are just saying other_thing === thing. The comparison is performed with the when expression as the lvalue.

This means I could rewrite the matches method as:

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def matches(compare_with)
  compare_with === @data_string
end

This also means it’s possible to be more flexible on the match:

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# @data_string = "coding for fun"
matches "oding"           # false
matches "coding for fun"  # true
matches /oding/           # true
matches String            # true

So, the next time you’re thinking of writing some code that needs to change based on class type or how something compares with something else, think if a case statement applies. If it does, see if === works to produce better code.